In general communication systems, signal transmission in wireless channel presents channel characteristics such as multipath information, doppler frequency shift, and phase rotation of received data. Therefore, when a receiver receives a signal, it estimates and uses current channel information to equalize the received data and restore the sent data.
In an orthogonal frequency division multiplexing (OFDM) system, some pilot sequences known to the receiver are transmitted on a time-frequency domain at intervals. For example, as shown in FIG. 1, the Long Term Evolution (LTE) system is provided with spaced pilot sequences, by which the receiver can restore channels on subcarriers and perform smoothing on the time-frequency domain so as to obtain channel estimation results on the whole time-frequency domain by common methods such as the Wiener filtering method, the simple linear interpolation method or FFT method with time-frequency domain transformation.
However, in the LTE system, beam forming of smart antennas may cause discontinuous phases among subbands of the OFDM in transmission modes 7 or 8. Therefore, in the situation of smoothing among subbands, the channel estimation error can be significant and the subbands may become narrow. For example, in the LTE channel, a resource block generally presents a discontinuous channel (in LTE, one resource block includes 12 subcarriers, and there are 3 or 4 UE-RS pilot frequencies thereon); in the situation of smoothing by a traditional filter in the resource block, the error may also be significant due to the edge effect of the filter.
In an OFDM receiver, channel smoothing is performed on the estimated channel in order to reduce the effects of noise on the estimated channel, thereby improving the system packet error performance. FIG. 2 illustrates a single stream OFDM transmitter 202 accepting an input stream s1 204 to a baseband encoder 206 which encoded stream is provided to an inverse fast Fourier transform (IFFT) 208 to produce a plurality of baseband subcarriers such as 2 through 2024 or 2 through 512and the subcarriers are modulated to a carrier frequency for coupling to an antenna 212 as transmitted signal X. The transmitted signal X is coupled through a channel with a frequency dependent characteristic H to receive antenna 232 of receiver 230 to form received signal Y=HX. The receiver 230 receives signal Y, which is baseband converted using RF Front End 233 and applied to FFT 234 to channel compensator 238 and to decoder 240 which generates the received stream S1′. Channel estimator 236 estimates the channel characteristic H during a long preamble interval, and the channel characteristic H is applied to channel compensator 238.
FIG. 3A illustrates a Multiple Input Multiple Output (MIMO) receiver 340 operative on two transmit streams s1 and s2 304 encoded 306 and provided to first stream IFFT 308 which generates baseband subcarriers, which are provided to RF modulator and amplifier 310 and coupled as X1 to antenna 316. Second stream IFFT 312 and RF modulator and amplifier 314 similarly generate subcarriers which are upconverted and coupled to antenna 318 as X2. Receiver 340 has three antennas 342, 344, 346, which couple to receivers 348, 350, 352 and to output decoder 354 which forms decoded streams s1′ and s2′. Each receiver 348, 350, 352 performs the receive functions as described for FIG. 2, however the channel estimation function 349, 351, 353 for each receiver uses the long preamble part of the packet to characterize the channel from each transmit antenna 316, 318 to each receive antenna 342, 344, 346. For example, receiver 348 must characterize and compensate the channel h11 from 316 to 342 as well as channel h21 from 318 to 342. Each channel characteristic h11 and h21 is a linear array containing real and imaginary components for each subcarrier, typically 1 through 1024. The channel estimator 349 therefore contains h11 and h21, estimator 351 contains h21 and h22, and channel estimator 353 contains h31 and h32. The 2.times.3 MIMO case of FIG. 3A shows the case where the number of remote transmitters Nt=2 and the number of local antennas and receivers Nr=3. For a MIMO receiver where the number of remote transmitters is Nt and the number of local antennas and receivers is Nr, the Nt*Nr channels have a frequency response which may be smoothed over a range of subcarrier frequencies using a finite impulse response (FIR) filter for I and Q channels. Such a channel smoothing filter would require a total of 2*Nt*Nr filters. For a 13 tap FIR filter, each tap would have an associated multiplier, so such an implementation would require 13 complex multipliers for each filter, or 26*Nt*Nr multipliers total at each receiver station.
It is to be appreciated that communicaiton interfaces can have other MIMO configurations. FIG. 3B is a simplified diagram illustrating various types of MIMO configuration.
Accordingly, due to channel discontinuity, the smoothing processing will result in significant errors in the channel estimation results. These errors affecting the performance, and signal quality. Thus, a method for obtaining more accurate channel estimation results is needed.